Metal Fatigue Blamed In Wind Tunnel Accident

July 12, 1989|By CHARLES H. BOGINO Staff Writer

HAMPTON — An investigation into an accident that caused $3.2 million in damage to a NASA-Langley Research Center wind tunnel suggests that engineers reconsider whether the metal band and clamps that caused the problem are necessary.

If they are, the report says, Langley engineers should inspect them more often.

The clamps, which were supposed to hold insulation, broke loose from their retaining band inside the wind tunnel in January and crashed into the tunnel's fiberglass fan blades.

The accident caused an eight-month delay in getting the newest of the space agency's 40 wind tunnels into service.

The 200-foot-long, 48-foot-wide tunnel was completed in 1983. Its test section measures 8 by 25 feet.

Langley officials hope to install new fan blades next month and resume research at the facility in November, said Keith Henry, a spokesman for the center.

According to a NASA report of the investigation released Tuesday, "fatigue and fracture failure" of the 4-foot-diameter stainless steel band, the thick ness of sheet metal, caused the metal clamps to break free.

"Debris from the failed structures passed through the fan blades causing major damage to all 25 blades and minor damage to other internal tunnel components," according to a statement issued by the Langley center. "All of the fan blades in the propulsion system were damaged beyond repair and will be replaced."

No one was injured.

The wind tunnel, called the National Transonic Facility, was being used to test a model of a military aircraft at the time of the incident, shortly after 3 p.m. Jan. 18.

The tunnel simulates aerodynamic flight conditions at what are called "transonic" speeds, between 600 and 900 mph, just below the speed of sound. The speed had reached only 530 mph when the band broke.

The fan was operating at 550 revolutions per minute; it can get up to 600 rpm, said Edward Bruce, operations manager of the wind tunnel.

"It was running within its operating envelope," he said. "There was no problem with any other part of the system when this part just failed."

The tunnel uses super-cold gases, such as nitrogen, to allow more accurate flight simulation at high altitudes. Temperatures in the tunnel when in use range from 152 degrees Fahrenheit to minus 300 degrees. Other tunnels use helium to help simulate aerodynamic flight conditions.

The extreme temperatures helped contribute to the failure of the stainless steel band, said the head of the team that investigated the incident, Joseph Yuska, deputy director of engineering at NASA's Lewis Research Center in Cleveland.

He said the temperature variation caused the metal to expand and contract rapidly and often.

"The stresses were very high, and I think the mechanical loads on it were higher than anticipated," said Yuska.